Transformer apparatus

ABSTRACT

Systems, apparatuses, and methods are described for a transformer supporting two or more sets of windings electrically connected to different voltage levels. Use of stress control materials or composite materials (comprising a matrix and filler) may direct electrical fields caused by the different voltage levels to have a lowered electrical field amplitude.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 63/074,139, filed Sep. 3, 2020. The contents of theabove identified application are incorporated herein by reference in itsentirety.

BACKGROUND

A transformer is an electronic device that includes a core and at leasttwo sets of windings of an electrical conductor, sometimes referred toas primary and secondary sets of windings. The core comprises a magneticmaterial with high magnetic permeability, such as a ferromagnetic metalsuch as iron, or ferrimagnetic compounds such as ferrites. The highmagnetic permeability of the core material, relative to the surroundingair, causes the magnetic field lines to be concentrated in the corematerial. The core may be shaped to enclose at least some of themagnetic field by forming a substantially closed loop of the corematerial, typically comprising two or more legs around with the windingsare wound. The sets of windings both comprise conducting wires that arewound around At least one core leg. The primary winding may control themagnetic fields within the core and the secondary winding may convertthe electrical field within the core to a voltage, possibly differentfrom the voltage across the primary winding. The transformer may providegalvanic isolation and voltage conversion between the first and secondsets of windings. When the number of windings in the first set ofwindings are different from the number of windings in the second set ofwindings, the transformer provides voltage conversion.

SUMMARY

The following summary presents a simplified summary of certain features.The summary is not an extensive overview and is not intended to identifykey or critical elements.

Systems, apparatuses, and methods are described for a transformer. Thetransform may comprise at least two sets of windings and a magneticcore. At least one of the two sets of windings may be at a substantiallyhigher voltage relative to the surrounding components. The edges of thehigh-voltage windings may be surrounded by a stress control region. Thestress control region may be a region of stress control materials, suchas a stress control material with a high dielectric constant, acomposite stress control material with capacitive or resistivestructures incorporated therein, or combinations thereof. Asemiconducting layer may be located near the high-voltage windings. Thestress control region may surround the edges of the semiconductinglayer. The electrical field stress control region may surround thecomponents of the transformer that are at a different voltage than thesurrounding structures, such as at a voltage difference of above about100 volts.

These and other features and advantages are described in greater detailbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

Some features are shown by way of example, and not by limitation, in theaccompanying drawings. In the drawings, like numerals reference similarelements.

FIG. 1A shows examples of transformers in accordance with the disclosureherein.

FIG. 1B shows an example model of a transformer in accordance with thedisclosure herein.

FIG. 1C shows a cross-section view of the example transformer of FIG.1B.

FIG. 2 shows an example rendering of additional details of a transformerin accordance with the disclosure herein.

FIG. 3 shows an example model of a transformer in accordance with thedisclosure herein.

FIG. 4 shows an example simulation of a transformer in accordance withthe disclosure herein.

DETAILED DESCRIPTION

The accompanying drawings, which form a part hereof, show examples ofthe disclosure. It is to be understood that the examples shown in thedrawings and/or discussed herein are non-exclusive and that there areother examples of how the disclosure may be practiced. It is also notedthat like references in the various figures may refer to like elementsthroughout the application. Similar reference numbers may also connotesimilarities between elements. For example, it is to be understood thattransformer 110B shown in FIG. 1B may be similar to, or the same as,other transformers described and shown herein, and vice versa.Throughout the application certain general references may be used torefer to any of the specific related elements. For example, transformer110 may refer to any of the various transformers, core 106 may refer toany of the various cores, electrical field stress control regions 103may refer to any of the various electrical field stress control, etc.The term “windings” may refer to a set of windings each comprising oneor more turns of a conductor surrounding a leg of a transformer core.

Reference is now made to FIG. 1A, which shows example transformers 110in accordance with the disclosure herein. Transformers 110 comprisecores 106AA and 106AB and multiple sets of windings (which may be insidea housing 108 depicted in FIG. 1A). Windings and internal transformerlayers may comprise edges (e.g., an edge where the windings terminate),such as ends of the windings, sharp corners, edges of semiconductinglayers, and the like. The edges may produce a high electrical field whena component operating at a different voltage (relative to the edge) isnearby. When the voltage difference between any two components of thetransformer is high, such as a voltage difference above about 100 volts(V), discharges or arcs may be caused by the resulting high electricalfield (e.g., electric stress). Transformers 110 described herein mayhave one or more electrical field stress control regions 103Esurrounding the edges of the components that have voltage differences(also referred to as regions 103E for brevity). These regions 103E mayencircle the transformer windings or layers, but regions 103E are shownas specific examples on the near side of the visible regions of eachtransformer. The electrical field stress control regions 103E may limitthe electrical field strength created around the edges of thecomponents. The electrical field stress control regions 103E maycomprise a high dielectric constant material. For example, the regions103E may comprise a material with a high dielectric constant disposedaround the edges that controls the electrical field strength (andresulting electrical stress) around the edges. For example, the regions103E may comprise embedded particles of other materials. For example,the regions 103E may comprise electronic components or micro-components(such as particles that affect the electrical impedance surrounding theparticles).

The cores 106AA and 106AB of FIG. 1A or the core 106B of FIG. 1B maycomprise two or more legs, and the windings may encircle one or more ofthese legs. For example, core 106AA may comprise three legs (E-type coreof FIG. 1A), and the windings may encircle the middle leg. For example,core 106AB may comprise two legs (C-type core of FIG. 1A), and windingsmay encircle both legs. For example, a core 106 may comprise two legsand one or more windings may encircle each leg. The regions 103E shownare some examples configured according to the internal structures of thewindings and layers. Other configurations of number of legs and windingsmay be implemented with aspects of this disclosure.

Reference is now made to FIG. 1B, which shows a model of a transformer110B in accordance with the disclosure herein. For brevity, onlyhalf-windings are shown, but it is understood that there may be fullwindings surrounding the one or more legs of the core 106B. For example,the windings structure (typically wound on a bobbin structure) maysymmetrically surround a leg of the core 106B, with similar crosssections around the annular edge of the windings or internal layer. Thetransformer 110B may comprise multiple windings (e.g., a first set ofwindings, also referred to as first windings 101, and a second set ofwindings, also referred to as second windings 102) and a core 106B. FIG.1B may depict, in a cross sectional schematic, a cross sectionalcomponent, which may be paired with another component (e.g., anotherhalf) as indicated by the dashed line on the bottom. To reduce thefailure rate of the transformer, regions 103 (also referred to asregions 103 for brevity) that control electrical field stress may becreated around edges of the windings. To reduce the failure rate of thetransformer, the electrical field stress control regions 103 may becreated around edges of the semiconducting layers 104B, 105B. Regions103 may comprise an electrical field stress control material or acomposite material. For example, materials with a high dielectricconstant (high-k material) may be placed where the electric field ishigh, such as for example to completely or partially surround (e.g.,encapsulate at least in part or enclose at least in part) the edges ofthe second windings 102. For example, a composite material comprising anon-linear resistive material (such as zinc-oxide particles for example)in an elastomer matrix may be used to stress control the electricalfield in the regions 103 of high electrical field surrounding the edges.For example, capacitive elements may be embedded in the regions adjacentto the edges. For example, capacitive elements may be embedded inregions between the edges and low voltage components. For example, theembedded capacitive elements may produce a non-linear capacitive effectusing a gradient of materials or additives/components, such as agradient of ZnO particle concentrations where higher concentrations areat locations of higher electrical field stress (in the absence of stresscontrol regions, such as near the second windings 102 or semiconductinglayer 105B) and lower concentrations at locations of lower electricalfield stress (such as near the first windings 101 or semiconductinglayer 104B).

A first pair of stress control regions 103A and 103B may be disposedaround edges or other portions of the second windings 102 that may causehigh electrical field concentrations. The regions 103A and 103B may forman annular ring adjacent to the edge of the winding that surrounds thecore 106. High electrical field stress above a breakdown voltagethreshold in the regions 103A and 103B may cause damage from resultingsurface arcs. Disposing regions 103A and 103B adjacent to the ends ofthe second windings 102 may reduce surface arcs in the transformer 110B,such as when a high voltage is generated in the second windings 102.Disposing a second pair of stress control regions 103C, 103D around theends of the first windings 101 may reduce surface arcs in thetransformer 110B, such as when a high voltage is generated on the firstwindings 101. For example, the voltage difference between the innerwindings 101 or inner semiconducting layer 104 and the core material maybe greater than 100V and the resulting electrical field stress may causebreakdown of the insulating layers therebetween. An inner insulationlayer 107A may be used between windings 101 and 102, which may have theadvantage of preventing an electrical connection between the windings101 and 102. An outer insulating layer 107B may surround the windings101 and 102, semiconducting layers 104 and 105, and regions 103. A firstsemiconducting layer 104B may be adjacent to second windings 102, andmay have a substantially similar electrical voltage as the secondwindings 102 (such as a mid-voltage electrical voltage of up to 69kilo-volts [kV]). For example, a second semiconducting layer 105B may beadjacent to first windings 101, and may have a substantially similarelectrical voltage as the first windings 101 (such as an electricalvoltage varying between zero and 100 volts (V) along the length of thefirst windings 101). Second semiconducting layer 105B may comprise edgesadjacent to the first semiconducting layer 104B, and regions 103A and103B may be disposed between these edges. Semiconducting layers 104 and105 may be used to shield the windings 101 and 102 from other electricalfields. For example, semiconducting layers 104 and 105 may have theadvantage of evenly distributing charge over the surface of thesemiconducting layer 104 and 105, which may prevent a buildup ofelectrical charge.

As may be illustrated in FIG. 1B, core 106B may be of a rectangularshape, such as by having two legs and two shorter members connectingparallel legs to form a full magnetic path. Other arrangements (e.g.,square shapes with more or fewer legs, round shapes, etc.) are possible.As an example, core 106B may comprise two half shapes, and may comprisean air gap (e.g., less than about 5 millimeters) for preventing eddycurrents from forming in the core.

The electrical field stress control regions 103 (such as regions 103A,103B, 103C, 103D from FIG. 1B as with regions 103E from FIG. 1A) maycomprise capacitive material or particles, non-linear resistivematerials with a high relative permittivity, or materials with a highdielectric constant (also referred to as materials with high-k). Forexample, a material with a relative permittivity greater than or equalto about 10 may be suitable for stress control regions in a transformer.

The materials used for the semiconducting, or partially-conducting,layers 104 and 105 may be based on a material comprising a volumeresistivity between that of a conductor and an insulator. For example, asuitable range of volume resistivity may be between about 0.001ohm/meter and about 10 kilo-ohm/meter. The semiconducting layers 104 and105 may be measured using a semiconducting sheet resistivity. Forexample, a suitable range of sheet resistivity may be between about 0.01ohm/square and about 10 mega-ohm/square. Semiconducting layers 104 and105 may be a matrix comprised of polymers, copolymers, thermosets,thermoplastic, and/or the like. For example, the matrix may be comprisedof polycarbonate (PC), polyether ether ketone (PEEK), polyamide,polypropylene (PP), polyphenylene sulfide (PPS), acrylonitrile butadienestyrene (ABS), polybutylene terephthalate (PBT), polyethyleneterephthalate (PET), polyphenylene oxide (PPO), polyphenylene sulfide(PPS), polyvinyl chloride (PVC), polyamide (Nylon), silicone, epoxy,acrylic, or any other such material. The partial conductivity of amaterial may be intrinsic (e.g., material properties) and/or extrinsic,such as by the addition of a particular percentage conducting particles,such as carbon black or the like.

The high-k materials used for stress control regions 103 may, in somecases, be based on commercially available materials, such as electricalstress control tape.

Stress control regions 103 may comprise non-linear resistive materials,such as non-linear resistive putties or tapes. For example, nonlinearZnO/silicone composite materials may be used for stress control regions103. Stress grading materials may be used for stress control regions103. For example, a semiconductor material having graded concentrationsin a composite material (and may have spatial graded permittivity) maybe used for stress control regions 103. For example, gradedconcentrations of ZnO particles in a silicon rubber matrix may be usedfor stress control regions 103. Composites made of microvaristor filledsilicone may be used for stress control regions 103. For example,composites including zinc oxide (ZnO) may be used for stress controlregions 103. For example, composites including bismuth oxide (Bi2O3)additives may be used for stress control regions 103. For example, asilicone matrix may be used for composite stress control regions 103.

Stress control regions 103 may comprise composites including particles,such as silicon carbide (SiC). Stress control regions 103 may comprisepolymeric composites including resistive additives, such as carbonblack. Stress control regions 103 may comprise polymeric compositesincluding additives of blends of different oxides (e.g., BaTiO3, TiO2,SiO2, Fe3O4, mica, etc.). The additives may affect the electrical fielddeveloped between the different voltages of components of thetransformer 110, depending on the application and structure of thewindings. For example, additives may adjust conductivity and affect theelectric field. For example, additives may adjust impedance and affectthe electric field. For example, additives may be distributed in varyingconcentrations around the regions surrounding the edges of windings orsemiconducting layers 104 and 105 to prevent electrical field stressesand resulting material breakdown from negatively effecting theperformance of the transformer 110. For example, a voltage may developfrom one end of a set of windings to the other during operation, sodepending on the electrical design of the transformer the voltagedifferences between the low voltage primary windings (LV windings) andmedium voltage secondary windings (MV windings) along the length of thewindings may vary. For example, voltage along the MV windings (without asemiconducting layer) may vary from 0 volts one end to 44 kV at theother end, while the LV windings may vary from 0 volts at one end to 100volts at the other end. In this example the stress control regions maybe adjacent to the ends of the MV windings (between the MV windings andthe LV windings) and the stress control regions may include additives athigh concentrations adjacent to the MV windings where the electricalfield is higher.

Reference is now made to FIG. 1B, which shows a cross-section view ofthe transformer of FIG. 1B. The cross-section view shows the core leg106B at the center. The cross-section view shows the stress controlregion 103C adjacent to the edge of the inner windings 101 (notvisible), and surrounding the core leg 106B. The cross-section viewshows the semiconducting layer 105B in between the inner windings 101and the outer windings 102, and surrounding the core leg 106B. Thecross-section view shows the stress control region 103A adjacent to theedge of the outer windings 102 (not visible), and surrounding the coreleg 106B.

Reference is now made to FIG. 2 , which shows an example rendering of atransformer 110C in accordance with the disclosure herein. Transformer110C may comprise LV windings 201 and MV windings 202. An insulationlayer 207A covers the MV windings 202 on both sides and an innerinsulation layer 207B fills the space between semiconducting layers 104Cand 105C. Semiconducting layers 104 and 105, may cover the LV windings201 and MV windings 202. First semiconducting layer 104C and secondsemiconducting layer 105C are also referred to herein as MVsemiconducting layer 104C and LV semiconducting layer 105C,respectively. Stress control regions 203 of a high dielectric constantmaterial (also referred to as a high-k material) may be placed betweenthe LV semiconducting layer 105C and the MV semiconducting layer 104C.This may have the advantage of reducing the electrical field strength inthis stress control region below a threshold that may cause surfacearcing (e.g., arcing due to the LV semiconducting layer 105C beingadjacent to the MV semiconducting layer 104C and MV windings 202). Amagnetic core 106, as described above, may be adjacent to LV windings201. Stress control regions 203 may be similar to electrical fieldstress control regions 103E described above, such as both configured toreduce electrical field stress and comprising high-k materials orcomposite materials. Insulation layer 207 may be similar to insulationlayer 107 described above, such as both configured to insulate theelectrical components and comprising electrically insulating materials.

In the example transformers shown in FIGS. 1A—2, windings 101, 102, 201and 202 may have a single winding layer of conducting material.According to aspects disclosured herein, each of windings 101, 102, 201and 202 may comprise more than one layers of windings, each windinglayer comprising multiple individual turns of the conductor wound arounda leg of the core (e.g., tens, hundreds, or thousands of individualturns). Windings 101, 102, 201 and 202 may be formed using single-strandwire, or multi-strand wire (such as a Litz wire).

Reference is now made to FIG. 3 , which shows an example modeltransformer 110D, such as the example transformer 110C of FIG. 2 , inaccordance with the disclosure herein. LV windings 201 may be adjacentto a core 106, as described above, of the transformer 110C. An innerinsulation layer 207DA may separate the main LV windings 201 from the MVwindings 202. An LV semiconducting layer 105C may be adjacent to theinner insulation layer 207DA and may be electrically connected to LVwindings 201. For example, a stress control regions 203 with highdielectric constant (high dielectric constant material also referred toas a high-k material) may be arranged between the LV semiconductinglayer 105C and the MV windings 202. Stress control regions 203 may bearranged to separate a relatively low voltage of the LV semiconductinglayer 105C from a relatively high voltage of MV windings 202. Forexample, the difference between the voltage of LV semiconducting layer105C and the voltage of MV windings 202 may be between about 100 V toabout 100 kV. An MV semiconducting layer 104CA may be arranged adjacentto the inner insulation layer 207DA and adjacent to MV windings 202. Anouter insulation layer 207DB may surround the MV windings 202, stresscontrol regions 203 and LV semiconducting layer 105C.

An anti-tracking layer 306 may be used to prevent arc tracking along thesurface of the transformer 110. The anti-tracking layer 306 may comprisean insulating material that has high surface resistivity. The highsurface resistivity may be especially particularly resistant to surfacearc tracking in the presence of moisture, dirt, oil, or othercontaminants. For example, materials with high surface resistance undertracking conditions can be identified using the methods specified in IECstandard 60112 and ASTM D3638, for example a material classified asMaterial Group I according to IEC standard 60112 may be used. Forexample, a tape or layer of material nylon 66 with a comparativetracking index of at least about 598 volts may be used as ananti-tracking layer 306.

Windings 101, 102, 201 and 202 may be wound around a bobbin, where thebobbin fits over a core 106 leg and is used to wind the winding'sconductors around the bobbin before inserting the bobbin on the coreleg. Windings 101, 102, 201 and 202 may feature two or more voltageterminals 208 and 209 that may be connected to voltage terminals of apower source circuit (such as a full-bridge of transistors or diodes, orany other compatible type of power electronics circuit) or a loadcircuit. An inner semiconducting layer 105 (which may be secondsemiconducting layer 105B or LV semiconducting layer 105C) may bedisposed around the inner windings (such as first windings 101 or LVwindings 201). For example, the inner semiconducting layer 105 maypartially or completely encompass the inner windings 101 and 201. Theinner semiconducting layer 105 may be electrically connected to (e.g.,as a result of being manufactured together with, or later electricallyconnected to) inner windings 101 and 201, and may diffuse the electricalfields strengths between the windings 101 and 102 or 201 and 202. Theinner semiconducting layer 105 may be connected to a voltage terminal208A or 208B of the inner windings 101 or 201. This may result in theinner semiconducting layer 105 being electrically connected to the sameelectrical voltage as the first voltage terminal 208A or 208B of theinner windings 101 and 201.

Insulating material (such as inner insulation layer 107A or innerinsulation layer 207DA) may be disposed (such as by injection, wrapping,heat-shrinking, or the like) between inner semiconducting layer 105 andouter semiconducting layer 104. The outer semiconducting layer 104 andinner semiconducting layers 105 may be disposed around (such as bypartially or completely encompassing the inner insulating layer 107A or207DA). The outer semiconducting layer 104 may be second semiconductinglayer 104B or MV semiconducting layer 104C. The terms inner and outer asused herein mean proximal to the core leg or distal to the core leg inthe axial direction (from the core leg axis).

Outer windings, which may be second windings 102 or MV windings 202, mayencompass the outer semiconducting layer 104. Outer windings 102 or 202may each feature two or more voltage terminals 209A and 209B, with afirst one of the voltage terminals 209A or 209B electrically connectedto the outer semiconducting layer 104. One of the two or more voltageterminals 209A or 209B may be electrically connected to the outersemiconducting layer 104 to the same electrical voltage as the firstvoltage terminal of the outer windings 102 and 202. Inner windings 101or 201 may each feature two or more voltage terminals 208A and 208B,with a first one of the voltage terminals electrically connected to theinner semiconducting layer 105. One of two or more voltage terminals208A or 208B may set the voltage of the inner semiconducting layer 105to the same electrical voltage as the first voltage terminal of theinner windings 101 or 201.

Each of windings 101, 102, 201 and 202 may feature two or more terminalsfor connecting to voltage terminals external to the transformer 101. Theterminals are electrically connected to the ends of each winding and areconfigured to connect to a circuit that utilizes the transformer, suchas a load circuit connected to the outer windings 102 or 202 and a powersource circuit connected to the inner windings 101 or 201. For example,each set of windings 101, 102, 201 and 202 may each have two voltageterminals, one at each end of the winding. The voltage terminals may beconnected to a varying voltage having an amplitude (such as analternating current [AC] voltage having an amplitude of several volts,tens of volts, hundreds of volts, thousands, or tens of thousands ofvolts). Windings 101, 102, 201 and 202 may be magnetically coupled toone another respectively via core 106, and may also be magneticallycoupled to other windings (such as additional secondary windings).

Semiconducting layers 104 and 105 may be formed using semiconductingmaterial. For example, semiconducting material 104 and 105 may besemiconducting plastic, isolating plastic with a semiconducting coating,or other semiconducting materials. Semiconducting layers 104 and 105 maybe electrically connected to one another to form a combinedsemiconducting layer 104 and 105 over both legs of core 106. Thesemiconducting layers 104 and 105 may be manufactured (such as by beingcast) from a single component, or may be formed combiningseparately-manufactured components. For example, a single mold may beused for manufacturing several semiconducting layers 104 and 105. Thesemiconducting layers 104 and 105 may be shaped, at least in part, toform Rogowski profiles or other profiles, which may increase uniformityin an electrical field created between components at two voltages.

Inner windings 101 and 201 may be electrically connected (such as bydirect electrical connection) to a first electrical voltage. Outerwindings 102 and 202 may be electrically connected (such as by directelectrical connection) to a second electrical voltage (that may bedifferent from the first electrical voltage). For example, innerwindings 101 and 201 may have a voltage near a ground voltage, and outerwindings 102 and 202 may have a voltage that is about 100V, 1000V, 10kV, 20 kV, 50 kV, 69 kV, or an even higher voltage. Inner windings 101and 201 may be electrically connected to a varying voltage. For example,inner windings 101 and 201 may be electrically connected to a voltagevarying (such as, sinusoidally or as a square-wave) between, forexample, about −1 kV and +1 kV, −10 kV and +10 kV, −20 kV and +20 kV,−69 and +69 kV. Inner windings 101 and 201 may be electrically connectedto a varying voltage having an amplitude up to 69 kV. Aspects disclosedherein may be modified to reduce electrical field stress in high voltagetransformers such as transformers configured to reach voltages of up toabout 1 megavolt.

A voltage drop may exist between sets of windings (e.g., as a result ofthe different windings being electrically connected to different voltagelevels). As discussed regarding the numerical examples above, thevoltage drop may be relatively large—for example, tens, hundreds orthousands of kilovolts. Electrically connecting outer windings 102 and202 to the outer semiconducting layer 104, and electrically connectinginner windings 101 and 201 to the inner semiconducting layer 105, maycause the voltage drop to exist between the inner semiconducting layer105 and the outer semiconducting layer 104. The inner semiconductinglayer 105 may encompass at least in part the inner windings 101 and 201and the outer semiconducting layer 104 may encompass at least in partthe inner semiconducting layer 105. This arrangement may have theadvantage of shielding the outer windings 102 and 202, the innerwindings 101 and 201, and the semiconducting layers 104 and 105 from oneanother. This shielding may reduce the insulation around the wires usedfor the windings to a rating that may be far less than the voltagedifference between outer windings 102 and 202 and inner windings 101 and201. For example, outer windings 102 and 202 may have a voltage drop ofup to about 1000V between two terminals on outer windings 102 and 202.Similarly, inner windings 101 and 201 may have a voltage drop of up toabout 1000V between two terminals on inner windings 101 and 201. Outerwindings 102 and 202 may be electrically connected to about 20 kV, andinner windings may be electrically connected to a ground voltage (e.g.,about 0V).

Insulating material layers 107 and 207 may be the same as the materialused for manufacturing a transformer case, and may be injected duringthe formation of the transformer 110. The injection may be, for example,vacuum potting, automatic pressure gelation, or other suitable methodsof injection.

Reference is now made to FIG. 4 , which shows an example simulation of atransformer 110E (such as the model transformer 110D of FIG. 3 ) inaccordance with the disclosure herein, according to a profile view, andincluding electric field indications 409. The example simulation wascomputed based on a 0 V voltage on the LV windings 201, a 30 kV voltageon the MV windings 202, and stress control regions 203 with a relativepermittivity of 25. Solid arrows indicate electrical field directions,with the size of the arrows indicating magnitudes. Short arrows indicatea relatively weak field, and longer arrows a relatively stronger field.The simulation indicates how the electrical field is reduced in theregion between the MV windings 202 and LV windings 201. Dashed arrowsindicate the field direction and strength in the absence of a stresscontrol region 203, where the longer dashed arrows indicate a strongerfield strength. As in FIG. 3 , FIG. 4 shows other elements of thetransformer 110E: inner insulation layer 207DA, LV semiconducting layer105C, MV semiconducting layer 104C, outer insulation layer 207DB, andanti-tracking layer 306.

Although examples are described above, features and/or steps of thoseexamples may be combined, divided, omitted, rearranged, revised, and/oraugmented in any desired manner. Various alterations, modifications, andimprovements will readily occur to those skilled in the art. Forexample, legs of core 106 may have round or oval cross-sections, ratherthan a rectangular cross-section; and outer semiconducting layer legsmay have round or rectangular cross sections instead of an ovalcross-section. As another example, core 106 may include more legs (e.g.,a third leg or more), and each leg may feature more than two sets ofwindings and/or more than two semiconducting layers. Such alterations,modifications, and improvements are intended to be part of thisdescription, though not expressly stated herein, and are intended to bewithin the spirit and scope of the disclosure. Accordingly, theforegoing description is by way of example only, and is not limiting.

The invention claimed is:
 1. An apparatus comprising: a magnetic corecomprising a core leg; inner windings disposed around the core leg;outer windings disposed around the inner windings, wherein the outerwindings comprise outer winding edges; an insulation layer between theinner windings and the outer windings, wherein the insulation layerextends beyond the outer winding edges and shields the outer windingsfrom the inner windings; an inner semiconducting layer between the innerwindings and the insulation layer; an outer semiconducting layer betweenthe insulation layer and the outer windings, wherein the outersemiconducting layer comprises outer semiconducting layer edges alignedwith the outer winding edges; and stress control material different froman insulating material of the insulation layer, wherein the stresscontrol material is disposed adjacent to the outer winding edges andadjacent to the insulating material of the insulation layer.
 2. Theapparatus of claim 1, wherein the stress control material comprises amaterial with a relative permittivity greater than or equal to ten. 3.The apparatus of claim 1, wherein the stress control material comprisesa non-linear resistive material.
 4. The apparatus of claim 1, whereinthe stress control material comprises a capacitive material.
 5. Theapparatus of claim 1, wherein the inner windings comprise a plurality ofinner voltage terminals, and wherein a first voltage terminal of theplurality of inner voltage terminals is electrically connected to theinner semiconducting layer.
 6. The apparatus of claim 1, wherein thestress control material comprises a composite material comprising amatrix and filler, and wherein the filler comprises a capacitivematerial.
 7. The apparatus of claim 1, wherein the stress controlmaterial comprises a composite material comprising a matrix and filler,and wherein the filler comprises a non-linear resistive material.
 8. Theapparatus of claim 1, wherein the outer windings comprise a plurality ofouter voltage terminals, and wherein an outer voltage terminal of theplurality of outer voltage terminals is electrically connected to theouter semiconducting layer.
 9. The apparatus of claim 1, wherein thestress control material comprises a composite material comprising amatrix and filler, and wherein the filler comprises a material withrelative permittivity greater than or equal to ten.
 10. The apparatus ofclaim 1, wherein an inner voltage terminal is electrically connected tothe inner semiconducting layer, wherein an outer voltage terminal iselectrically connected to the outer semiconducting layer, wherein theinner voltage terminal is configured to operate at a first voltage, andwherein the outer voltage terminal is configured to operate at a secondvoltage.
 11. The apparatus of claim 10, wherein, when the apparatus isoperational, the first voltage is less than the second voltage by morethan 100 volts.
 12. The apparatus of claim 10, wherein, when theapparatus is operational, the first voltage is less than the secondvoltage, and the first voltage and the second voltage comprise analternating voltage.
 13. The apparatus of claim 10, wherein, when theapparatus is operational, a voltage difference between the first voltageand the second voltage varies according to an alternating voltage havingan amplitude up to 69,000 volts.
 14. The apparatus of claim 10, wherein,when the apparatus is operational, the first voltage is an electricalground potential.
 15. The apparatus of claim 10, wherein, when theapparatus is operational, the second voltage is greater than 5,000volts.
 16. The apparatus of claim 1, wherein the apparatus is atransformer.
 17. A power device comprising: a magnetic core comprising acore leg; inner windings disposed around the core leg; outer windingsdisposed around the inner windings, wherein the outer windings compriseouter winding edges; an insulation layer between the inner windings andthe outer windings, wherein the insulation layer extends beyond theouter winding edges and shields the outer windings from the innerwindings; an inner semiconducting layer between the inner windings andthe insulation layer; an outer semiconducting layer between theinsulation layer and the outer windings, wherein the outersemiconducting layer comprises outer semiconducting layer edges alignedwith the outer winding edges; and stress control material different froman insulating material of the insulation layer, wherein the stresscontrol material is disposed adjacent to the outer winding edges andadjacent to the insulating material of the insulation layer.
 18. Thepower device of claim 17, wherein the power device is a transformer. 19.A system comprising: a magnetic core comprising a core leg; innerwindings disposed around the core leg; outer windings disposed aroundthe inner windings, wherein the outer windings comprise outer windingedges; an insulation layer between the inner windings and the outerwindings, wherein the insulation layer extends beyond the outer windingedges and shields the outer windings from the inner windings; an innersemiconducting layer between the inner windings and the insulationlayer; an outer semiconducting layer between the insulation layer andthe outer windings, wherein the outer semiconducting layer comprisesouter semiconducting layer edges aligned with the outer winding edges;stress control material different from an insulating material of theinsulation layer, wherein the stress control material is disposedadjacent to the outer winding edges and adjacent to the insulatingmaterial of the insulation layer; a power source circuit connected tothe inner windings; and a load circuit connected to the outer windings.20. A method comprising: providing a magnetic core comprising a coreleg; disposing inner windings around the core leg; disposing outerwindings around the inner windings, wherein the outer windings compriseouter winding edges; disposing an insulation layer between the innerwindings and the outer windings, wherein the insulation layer extendsbeyond the outer winding edges and shields the outer windings from theinner windings; disposing an inner semiconducting layer between theinner windings and the insulation layer; disposing an outersemiconducting layer between the insulation layer and the outerwindings, wherein the outer semiconducting layer comprises outersemiconducting layer edges aligned with the outer winding edges; anddisposing stress control material, different from an insulating materialof the insulation layer, adjacent to the outer winding edges andadjacent to the insulating material of the insulation layer.